High-frequency circuits are commonly used in modern applications such as wireless communication applications. A common problem faced by designers is the signal loss in the substrates that are underlying the high-frequency circuits, which signal loss is partially caused by the parasitic capacitors formed between the high-frequency circuits and the underlying substrates. Typically, with the increase in the frequency of the signals, the loss also increases. This significantly limits the design of high-frequency circuits.
FIG. 1 illustrates conventional radio frequency (RF) transformer 4 formed over semiconductor substrate 2. Dielectric layer(s) 8 separates RF transformer 4 from semiconductor substrate 2. RF transformer 4 forms parasitic capacitors 6 with semiconductor substrate 2. The undesirable parasitic capacitors 6 limit the operation frequency of RF transformer 4 to about 10 GHz and below. When the frequency is further increased, the signal loss in semiconductor substrate 2 significantly increases.
The signal loss also occurs to other integrated circuit devices when these devices are operated under high frequencies. For example, FIG. 2 illustrates junction diode 12, which is formed between P+ diffusion region 14 and N-well region 16. P+ diffusion region 14 and N-well region 16 are both formed in substrate 18. Again, parasitic capacitors 20 are formed between P+ diffusion region 14, N-well region 16 and substrate 18, and between substrate 18 and metal lines 22 that are connected to junction diode 12, and the like. Parasitic capacitors 20 result in significant signal loss in the signal carried by junction diode 12, which loss may be as severe as about 4 to 5 decibels. Similarly, other diodes such as FinFET diodes and Schottky diodes also suffer from similar problems.
Further, the signal loss problem may be worsened by the increasing down-scaling of integrated circuits, which causes the distances between the high-frequency devices and the respective underlying substrates to be increasingly smaller. The reduced distances result in the increase in parasitic capacitances. Solutions are thus needed to solve the above-discussed problems.